In the recovery of oil from oil-bearing reservoirs, it is usually possible to recover only a minor portion of the original oil present in the reservoir by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is fluid flooding which involves the injection of a fluid, such as water or a miscible solvent into an oil-bearing reservoir. As the fluid moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that a variety of factors such as the interfacial tension between the injected fluid and the reservoir oil, the relative mobilities of the reservoir oil and the injected fluid, and the wettability characteristics of rock surfaces within the reservoir are factors which influence the amount of oil recovered by the various flooding techniques. Thus, it has been proposed to add surfactants to injected water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add thickening agents to all or part of the injected fluid in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected fluid and the oil and improving the sweep efficiency of the flood.
In fractured formations, the fractures generally act as primary fluid flow conduits, with the reservoir matrix providing the primary fluid storage capacity for the reservoir fluids. If the fractures are highly conductive, as compared to the reservoir matrix, the injection of a displacement fluid, such as water, may be relatively ineffective in recovering oil from the matrix. The capability of the displacing fluid to enter the matrix to displace oil has been found to depend in large part on the wettability of the matrix.
Wettability is the tendency of a surface to be wetted by a fluid. Water will "wet" or spread on a water-wet solid surface and will be repelled by an oil-wet surface. When an oil-filled porous rock matrix containing a connate water saturation is water-wet, water will spontaneously imbibe into the matrix to displace oil. However even after the waterflood, oil remains in the matrix, either because of poor sweep characteristics or because it remains trapped by capillary forces. Such trapped oil is often referred to as residual oil, and is typically 20 to 35 percent of the total pore volume. Unswept pore space resulting from the presence of the fracture may leave an additional 10 to 20 percent of the oil within the matrix. A method often employed to recover such residual oil after a waterflood is to utilize a miscible solvent, such as a light hydrocarbon, carbon dioxide or nitrogen. This is known to those skilled in the art as tertiary recovery by miscible flooding. In a water-wet fractured formation, the miscible solvent will not imbibe into the matrix and therefor will tend to exhibit poor flow and sweep of the formation.
In contrast to the water-wet formation, an oil-wet, fractured formation will experience poor oil recovery by waterflooding because the water will not imbibe into the matrix. The same oil-wet formation will experience good oil recovery by miscible solvent flooding, however.
Therefore what is needed is a process for recovering oil which enhances the efficiency of secondary and tertiary oil recovery techniques in formations having natural fractures therein.